Conventionally, in digital copying machines, facsimile machines, etc., an image signal obtained by reading a document composed of a character region, a photographic region or a dot region, or a document composed of a mixture of these regions by means of a CCD image sensor is processed to attain an improved quality of a recorded image. In the described process of the image, first, the type of the read image is identified, i.e., whether the read image is a character image, a photographic image or a dot image. Then, based on the result of this identification, an optimum process is applied to each picture element.
Known image identification methods to attain an improved image quality in the described manner include: A method wherein an image is divided into blocks, each being composed of plural picture elements, and an identification of an image is performed per each block by a pattern matching technique or using feature parameters indicative of features of the character image or the dot image.
However, the image identification method using the pattern matching technique has the following problem. That is, since it is required to prepare many patterns, a huge memory is required. Thus, this method is not suitable for practical applications. For this reason, the image identification method using the feature parameters is used in many cases.
Japanese Laid-Open Patent Publication No. 194968/1986 (Tokukaisho 61-194968) discloses a dot/photographic image identification method as an example of such image identification method using the feature parameters. In the method of this disclosure, a change in signal level in two picture elements which are three-dimensionally in sequence is measured separately between the following two cases: the case where the two picture elements are in sequence in a main-scanning direction and the case where the two picture elements are in sequence in a sub-scanning direction. Then, a sum of the measured values in each block is compared with a predetermined threshold, and an image is recognized based on the result of this comparison.
Japanese Laid-Open Patent Publication No. 147860/1987 (Tokukaisho 62-147860) discloses an intermediate tone facsimile signal processing method as another example of the image identification method using the feature parameters. In this method, first a difference between a maximum signal level and a minimum signal level in the block is computed, and the computed difference is compared with a predetermined threshold. As a result of this comparison, if the computed difference in signal level is smaller than the predetermined threshold, an identification signal for a smooth change in signal level indicating that the block is located in a photographic region is outputted. On the other hand, when the computed level difference is larger than the predetermined threshold, an identification signal for a sudden change in signal level indicating that the block is located in the character region or on the outline portion of the photographic image is outputted. Furthermore, according to a predetermined access order in the block, a number of changes in signal level in two picture elements which are three dimensionally in sequence is compared with a predetermined number. As a result of this comparison, if the number of changes is greater than the predetermined number, an identification signal indicating that the block is located in the dot region is outputted. On the other hand, if the number of changes is smaller than the predetermined number, an identification signal indicating that the block is not in the dot region is outputted. Based on the identification signal, the signal process is applied to each picture element in the block.
For the image process to attain an improved image, conventionally, a spatial-filtering process is adopted. One example of the technique of attaining an improved image quality adopting the filtering process is disclosed in Japanese Examined Patent Publication No. 5-21384 (Tokukohei 5-21384). In the image processing apparatus of this disclosure, a spatial filter for smoothing the image signal and a spatial filter for enhancing the image signal are prepared beforehand, and based on an output from edge detection means for detecting an edge portion of the image signal, a signal obtained by applying a smoothing process to the image signal or a signal obtained by applying an enhancement process to the image signal, or a signal obtained by applying a combined process of the smoothing process and the enhancement process is outputted. Another method is adopted in a filtering apparatus disclosed in Japanese Laid-Open Patent Application No. 246076/1988 (Tokukaisho 63-246076). In this method, a spatial filter for removing a dot region is prepared beforehand, and when the edge portion is not extracted by edge extraction means for extracting the edge portion of the image signal, a signal having gone through the filtering process is outputted. On the other hand, when the edge portion is extracted, a signal is outputted before having gone through the filtering process.
However, in the described image identification method, an error may occur in identifying the image, and to attain a high quality image, a still more precise identification is required. The possible cause of this identification error of the image would be a mismatch of a feature parameter. That is, the error is caused as the feature parameter fails to fully represent the features of each region. The other possible cause of the described identification error is an inappropriate selection of the classification method itself for identifying the image based on the feature parameter, or the threshold value for the classification.
Moreover, in the conventional filtering process by the spatial filter prepared beforehand, an identification process is completely performed for each block based on the resulting feature parameters, and based on the result of identification, a filtering process is applied to a target picture element in block using a predetermined spatial filter. In this method, the image is greatly affected when an identification error occurs, and the feature parameter of a target picture element reflects only a specific filter feature, thereby presenting the problem that a fine processing such as applying an optimal filtering process to each picture element cannot be achieved.